Strain measuring instrument



Nov. 14, 1950 L. D. STATHAM 2,530,184

STRAIN MEASURING INSTRUMENT Filed Jan. 19, 1944 2 Sheets-Sheet 1 FIG. 1

INVENTOR. LOUIS D. STATHAM ATTORNEY L. D. STATHAM STRAIN MEASURING INSTRUMENT Nov. 14, 1950 "2 Sheets-Sheet 2 Filed Jan. 19, 1944 FIG. 2

FIG. 3

INVENTOR LOUIS D. STATHAM ATTORNEY Patented Nov. 14, 1950 STRAIN MEASURING INSTRUMENT Louis D. Statham, Beverly Hills, Calif., assignor to Curtiss-Wright Corporation, a corporation of Delaware.

' Application January 19, 1944, Serial No. 519,067

This invention relates to mechanism for elece trically measuring and indicating quantities which can be caused'to actuate slight variations in electrical resistance. It is particularly applicable to the measurement and indication of structural strains, suchas in aircraft, ships, land vehicles, bridges, highways, subways, towers, pressure vessels and tanks, dams, cranes and hoists, machinery, and ordnance, by means of resistancetype strain-gages According to one embodiment of the present invention, aplurality of strain gages, or' other resistance-type electrical measuring instruments, are incorporated in a plurality of Wheatstone bridges. The current outputs from these bridges are selectively measured and indicated, by means of mechanism hereinafter described, on a galvanometer or other device for indicating electric currents. overload relay, as described hereinafter, and other protective devices are incorporated in the system for the purpose of preventing excessive current loads on the galvanometer.

This system and its operation may be best understood by reference to the accompanying drawings, in which:

Figure 1 is a wiring diagram-and arrangement of an embodiment of this invention, containing five strain gage circuits; Figure 2 is a front view of an instrument case and panel for use with the circuits shown in Figure 1, but in which 48 strain gage circuits are incorporated; and Figure 8 is a simplified wiring diagram of a portion of the mechanism shown in Figure 1.

Referring now to Figure 1, five Wheatstone bridge circuits are indicated generally at II, I2, I

I3, I4, and I5, respectively. The bridge circuit indicated at II is representative of the others and will be described in detail. It consists of two fixed resistances I1 and I8, of 120 ohms each,

known as the working gage, is connected across the terminals 19' and 20, and the other gage I0, known as the compensating (or"control) gage, is connected across the terminals 2I and 22. The

gage 9 may, for example, be bonded to an element 7 8 which may be subjected to various stresses with accompanying strain of the gage 9. The input battery is connected through the leads 23 and 24, respectively, as hereinafter described. The galvanometer is connected through leads-25 and 26,

. 8 Claims. (Cl. 73- 885) also as hereinafter described. Also incorporated in the bridge are two channel balancing arms shunted across the terminals I9 and 20 of the working gage and 2| and 22 of the compensating gage, respectively. These channel balancing arms are composed of two fixed resistances 27 and 28 of 10,000 ohms each and a potentiometer-connected rheostat or potential divider 29 of 20,000 ohms resistance. The variable contact arm 30 of the potentiometer 29 is connected to the terminals 30 and 2 I, so that movement of the contact arm 30 increases the resistance shunting one of the gages and decreases the resistance shunting the other. By this means it is possible to compensate for a different of resistance of almost 1 ohm between the working and compensating gages.

Terminal 22 and resistance I8 are both connected through lead 24' and the lower deck 41 of a 3-deck main switch 50 to the positive terminal of'a 6-volt battery I6. Terminal I9 and resistance I! are likewise both connected through lead 23 and lower deck of alt-deck channel selector switch 34 (the other two decks of which are indicated at 35 and 31), circuit 31, parallel resistances38 and 39, and circuit 32 to the negative terminal of the 6-volt battery I6.

Resistance 38 is a fixed resistance of 35 ohms, while resistance 39 is a variable resistance or rheostat of'50 ohms. Operation of the rheostat 39 varies the voltage applied to the bridge and thus the current output per ohm of bridge unbalance. As strain gages are manufactured of various materials, the amount of strain (in inches per inch) necessary to secure a certain variation in resistance varies with the nature of the material, according to the following formula:

AR :kRS (1) EAR - where A is the current in amperes flowing through the galvanometer circuit, E is the electromotive force in volts of the battery I6, G is the resistance described in greater detail).

in ohms of the galvanometer circuit, and B is the resistance in ohms of the battery circuit. Transposing:

AR=%(R+G) (R-l-B) and substituting from (1) and dividing by AlcR:

be a constant, or an integral multiple thereof, for all types of strain gages used, it is easily seen that this can be readily accomplished by suitable adjustment of B for varying values of k. The constant It is known as the gage factor, and ordinarily varies between 2.0 and 2.15 for commercial strain gages now on the market. The rhe'ostat 39 and the fixed resistance 38, by varying the resistance, thus-acts as an adjusting means or"gage factor control, for securing a constant value of The junction between resistances I! and I8 is connected through circuit 25 and the middle deck 35 of the channel selector switch 34 to circuit 46, the latter in turn connecting directly to one terminal 4-2 of the galvanometer 4|. Terminals 20 and 21' are connected through circuit 26, the upper deck 310i the channel selector switch and circuit 33 to a relay switch H6 (as hereinafter The opposite pole of the relay switch H6 is connected to a circuit 44 which is in turn connected, through the multiplying switch system as described herein, and circuit 45, to the second terminal 43 of the galvanometer 4!. The'galvanometer contains an internal resistance of 53 ohms, and circuit 45 is in series with an external resistance 46 of 190 ohms. An external shunt resistance 4'! of 165.5 ohms is connected across circuits 46 and 45.

The multiplying switch system is indicated generally at I66. This enables low strain values to be read with accuracy on a galvanometer scale of limited range, and at the same time permits the reading of higher strain values outside this range, by reducing the current (equivalent to a given strain) sent through the galvanometer to a fixed fraction (such as one-half, one-fifth, or one-tenth) of its normal value. Stated in another way, the ratio of strain to current flowing through the galvanometer (S/C) is increased by a certain integral multiplying factor (e. g., 2, 5, or 10). With a multiplying factor of 5, for example, a gage reading of 120 micro-inches per inch would indicate an actual strain of 600 micro-inches per inch. From Equation 5, assuming E, k, R, and B are constant, the following relation holds:

8 R+G 0 F From Equation '7, the ratio can be increased by decreasing the value of F. For best results, the value of G (the total resistance of the galvanometer circuit) should be approximately the same whatever the multiplying factor used. By this is meant that the highest eifective value 'of G should preferably not exceed the lowest effective value of G by more than about 50 per cent. By this means, accurate measurement and proper control is greatly facilitated.

The multiplying switch system comprises a plurality of momentary contact switches, one for the normal range and one for each multiplying factor. In the form shown in the drawings there are four of these switches, indicated at 5|, 6|, 1|, and at, these being respectively for the normal range (or multiplying factor of 1), for a multiplying factor of 2, for a multiplying factor of 5, and for a multiplying factor of 10. These switches are so arranged, as hereinafter described, that actuation of two or more switches will cause only the one with the highest multiplying factor to be operative. Each switch is composed of at least two jointly operated movable ccntact arms 52, 62, 1 2, and 82, respectively, and 53, 63, i3, and 83,- respectively. In Figure 1, each switch is shown in its lower or inoperative position, contact arm 52 being in contact with circuit 54, arm 53 with circuit 55, arm '62 with circuit 64, arm 63 with circuit 65, arm 1'2 with circuit [4, arm 13 with circuit [5, arm 82 with circuit 64-, and arm 83 with circuit 85. Circuits 54 and 55 are open or dead-end, but circuits E4, 65, M, I5, 84, and are each connected to the corresponding contact arm of the switch having the next lower multiplying factor. Thus circuit 64 is connected with contact arm 52, 65 with 53, 14 with 62, 15 with 63, 84 with 12, and 85 with 13. The contact arms 82 and '83 of switch 8| are connected to each other by-means of circuit 58.

Each switch 5|, 6|, H, and 81 also has an upper or operative position. In these positions contact arm 52 is in contact with circuit 56, arm 53 with circuit 51, arm 62 with circuit 66, arm 63 with circuit 61, arm 12 with circuit 16, arm 13 with circuit 11, arm 82 with circuit 86, and arm 83 with'circuit 81.

Circuit 56 is connected directly to circuit 44 and circuit 51 directly to circuit '45. Thus it is seen that actuationof switch 5| will cause the current to fiow through circuit 44, circuit 56, arm 52, circuit 64, arm 62, circuit 14, arm 2, circuit 84, arm 82, circuit-58, arm '83, circuit 85, arm 13, circuit 15, arm 63, circuit 65, arm '53, circuit 51, and circuit 45. In this position, the resistance of the galvanometer circuit (shown in Figure 3) is merely the combination of the galvanometer internal resistance (-53 ohms), the external series resistance 46 (lohms), and the external shunt resistance 41 (165.5 ohms); Thus, in Figure 3, resistances HH and 1 62 are each zero and resistance I63 is infinity. It will be observed that, when switch 5! is closed, the circuit is broken if any of the remaining switches 6!, H, or 8| are in the upper or operative position.

In addition to the contact arms 62, 63, 12, I3, 82, and B3, switches 6|, H, and SI each have a resistance 9! of 40.75 ohms.

resistance 93 of 48.25 ohms. .is connected through a resistance 94 of 48.25

- circuit 45; served that. the circuit is not affected if any of I the remaining switches are in the upper or opera- -tive position. The remaining part of the current (starting --with circuit 58) will flow through. cir- 5, third. contact arm 68, I8, and 88, respectively, jointly operated with the other two. These contact arms 68, 18, andv B8 in their lower or inoperative positions merely rest against stops 59. In their upper or operative positions, however, they contact circuits 69, I9, and 89, respectively. Circuit 69 is connected to circuit 19 through a resistance 60 of 107 ohms, circuit 19 to circuit 89 through a resistance 10 of 28.75 ohms, and

circuit 89 to circuit 40 through a resistance 80 of 24.25ohms. The movable contact arms 68, 18,

and 88. are all directly connected to each other and, through a circuit- I09, to circuit 58. It is seen that no currentflows through circuit I09 unless one of the three switches 1|, or 8| is in the upper or operative position. It will also be seen that, substantially no current will flow through resistance 60 (even if switch BI is closed) .if switch-H is closed, since the current will follow the path of least resistance. Likewis'eysubvstantially no current will flow through resistances 60 or 10 (even if switches 6| and/or 1| are closed) if switch 8| is closed. 7

Circuit 56 is connected to circuit 44 through a Likewise circuit 51 is connected through a resistance 92 of 40.75 ohms to circuit 45. Thus it is seen that actuation of switch will cause the current to flow through circuit 44, resistance 9|, circuit 66, arm 62, circuit-14, arm 12, circuit 84, and arm 82. Thence,

part of the current will flow through circuit 58,

arm 83, circuit 85, arm 13, circuit 15, arm 03, circuit 61, resistance 92,- and circuit 45. In this arcuit is not affected if switch 5| is in the upper or operative position. The remaining part of the current (starting with circuit 58 will flow through circuit [09, circuit 69, resistances 60, 10, and 80, and circuit 40, thus shunting across the galvanometer.

Circuit 76 is connected to circuit 66 through a Likewise circuit ll resistance 94, circuit 61, resistance 92, and circuit 45. In this arrangement, it will be observed that ,the current is broken if switch 8| is in the upper 77 or operative position. On the other hand, the

circuit is not affected if either switch 5| or BI is in the upper or operative position. The remaining part of the current (starting with circuit .58)

"will flow through circuit I09, circuit 19, resistances 10 and 89, and'circuit 40, thus shunting across the galv'anometer.

Circuit 86 is connected to circuit 16 through a resistance 95 of 9.3 ohms. Likewise circuit 8'! is connected through a resistance 99' of 9.3 ohms 'to circuit Tl. Thus it is seen that actuation of switch 8| will. causethe current to flow through circuit 44, resistance 9|, circuit 06, resistance 03, circuit 15, resistance 95, circuit 86, arm. 82, circuit 58, 'arm 83, circuit 81, resistance 96, circuit Tl, resistance 94, circuitv 61', resistance 92, and

In this arrangement it will be obcuit. I09, circuit 89, resistance 80, and circuit 40, thus shunting across the galvanometer.

Referring now to Figure 3, the resistances in the galvanometer circuit are illustrated in a simplified fashion. Resistance I04 is the internal resistance of the galvanometer. Resistance I02 is none, 1, 2, or 3 of the resistances 9|, 93, and 95. Resistance |0| is none, 1, 2, or 3 of the resistances 92, 94, and 96. Resistance I03 is none, 1, 2, or 3 of the resistances 60, 10, and 80. The remaining reference characters are the same as in Figure 2. The internal resistance I04 of the galvanometer in all switch arrangements is 53 ohms. The external series resistance 45 for all switch arrangements is 190 ohms. The external shunt resistance 4? for all switch arrangements is 165.5 ohms. When switch 5| is in operative position and switches 6|, 1|, and 8| in inoperative positions, resistances NH and I02 are substantially zero, and resistance I03 substantially infinite. When switch 6| is in operative position and switches TI and 8| in inoperative position, resistances |0| and H12 are 40.75 ohms each, and resistance. I03 is ohms. When switch H is in operative position and switch 8| in inoperative position, resistances |0| and I02 are 89 ohms each, and resistance 103 is 53 ohms. When switch 8| is in operative position, resistances |0| and I02 are 98.3 ohms each, and resistance I03 is 24.25

ohms. With these circuits, the effective values of G (in ohms) and F (Equation 5) are shown in the following table:

Operative R+G Switch G F F Itis seen that the factors R G F .for operation of switches 5|, 6|, H, and 8| are in' the respective proportions of approximately 1, 2, 5, and 10, since, with other factors constant, the strain S, requiredto produce a given galvanometer scale reading as a direct function of.

the current C, is twice as great for switch 6| as 'for switch 5|, five timesas great for switch 1| as for switch 5|, and ten times as great for switch .Sl as for switch 5|. "1 Referring further to Figure 3 and the table 'above,'it will be noted that the arrangement of resistances is such as to'give a fairly small range of values for G, the total resistance of the gala period of one second. I

The galvanometer scale is calibrated directly in microinches of strain per inch, with switch 5| in operation. With switches 6|, 1|, or 8| in operation, the gage reading is multipliedby 2, 5, or 10, respectively. The apparatus is calibrated at periodic intervals, principally to compensate for changes in internal resistance of the battery 16. 'Calibration-is' accomplished by means of the mush-button switch 91. The upper-deck 98 0f this switch shunts a resistance I of 50,000 ohms across the terminals 2I and 22, thus unbalancing the bridge by 0.297 ohm. The lower deck 99 of the switch 91 connects circuit 45 directly with circuit 44, thus putting a load on the galvanometer equivalent to that accomplished by operation of the switch I. With a gage factor of 20, this should give a galvanometer reading of 1237.5 microinches per inch. Proper reading is obtained by manipulation of the rheostat 39. For convenience in operation, all of the strain gages used at any one time should be of the same gage factor, so that the same setting of rheostat 39 may be used on all channels.

Referring now to switch III] in the galvanometer circuit, it is normally kept in the position shown in the drawing by gravity or by means of a light spring. A solenoid III located adjacent switch I I0 is normally deenergized, but when energized serves to open switch IIIl. Solenoid III is electrically connected at one terminal to a circuit II2 which leads through two resistants I I3 and H4 to the two anodes or plates I23 and I24 respectively of two gas-filled electronic emission tubes I2! and I22 respectively, for purposes hereinafter described. At its other terminal solenoid III is connected to a push button reset switch II5, the latter being connected through upper deck 49 of main switch 50 to the positive terminal of a 145 volt battery I IT. The negative terminal of the battery I I1 is connected through a circuit II8 to the positive terminal of a 4.5 volt battery I40. Circuit I I8 is also connected to a circuit I20, the main connection of the latter circuit I being to the cathodes I and I25 of the tubes I2I and I22 respectively. Thus it is seen that, if a current flows through either of the tubes I2I or I22, the solenoid III is actuated and the switch H0 is opened, so that no current flows through the galvanometer. The tubes I2I and I22 also contain operating grids I21 and 628 respectively, screen grids I 29 and I respectively, and heaters I and I36 respectively.

Current does not flow between the cathodes I25 and I26 and the plates I23 and I24 unless the operating grids I21 and I28 are at a sufliciently high potential. These operating grids are connected through resistances I3I and I32 of 10,000 ohms each to circuits and 33 respectively. In order to prevent wide fluctuations of potential between circuits 33 and 40 and cathodes I25 and I26, two resistances I01 and I08 of 10,000 ohms each are connected in series across circuits 33 and 40, and the connections between the two resistances joined to circuit I20 by means of circuit I06. With this arrangement, if there is a current output from the bridge through the galvanometer circuit, it makes no difierence in which direction it flows, since either grid I2I or I28 will be excited to a positive potential. The excitation potential at which control grid I2! or I28 will cause a current to flow between cathode I25 or I26 and plate I23 or I24 is such that values below this will not cause excessive currents to flow through the galvanometer when switch IsI is operative. This excitation potential is controlled by means of screen grids I29 and I30, which are connected through two 5000 ohm potentiometers I33 and I34 respectively, circuit I42 and middle deck 43 of main switch 50 to the negative terminal of battery I40. As the positive terminal of battery I40 is connected through circuit I20 to the cathodes I25 and I26, and as there is also a shunt I4I around, thetubes and between circuits I20 and I42 and including potentiometers I33 and I34, it is seen that the potential difference between screen grids I29 and I30 and cathodes I25 and I23 may be accurately controlled by means of potentiometers I33 and I34. After the apparatus is once adjusted, the setting of these potentiometers should not normally be changed.

The heaters I35 and I36 of the tubes I2I and I22 are connected across circuits 24 and 32, by means of circuits I31 and I38, to the battery I6. Circuit I3'I is also connected to a switch I39 which is normally open, but which is actuated to the closed position by means of solenoid III. A circuit I43, in which is located a warning light I44, connects the opposite pole of the switch I39 to circuit I38. Switches III) and I39 conveniently form the two decks of a two-deck switch.

A light II6 connected around the battery I6 and lower deck 41 of switch 50 indicates when'the apparatus is in operation.

Referring now to Figures 1 and 2, the apparatus is prepared for use by connecting a plurality of strain gages and compensating gages across the terminals I9 and 20 and 2I and. 22 respectively of bridges II, I2, I33, I4, I5, etc. All these gages should be of the same gage factor. Main switch 50 is then closed, lighting the light H6. If the warning light I44 goes on, it indicates trouble, and the system should be checked until the trouble is located. Since the galvanometer circuit is open when the warning light is on, it must be closed by deenergizing solenoid III, which can be accomplished by opening the reset switch II 5. If the warning light goes on again after the reset switch II5 is closed, it indicates that trouble is still present.

Channel selector switch 34 is then set at bridge position II, and switch 8| closed. If the galvanometer 4| shows a reading in either direction, the bridge is balanced by manipulating the potentiometer 29 corresponding to that bridge position until the galvanometer reads 0. The adjustment may be made more accurate by opening switch 8I and closing switch 5|. The same operation is then repeated for the remaining bridges I2, I3, I4, I5, etc. The apparatus is calibrated by setting the channel selector switch 34 at any balanced bridge position, closing the calibration switch 9?, and manipulating rheostat 39 to secure the proper calibration adjustment.

After the bridges are balanced and calibrated, any desired loads are put on the strain gages. Strains are read by moving the channel selector switch to successive bridge positions, closing switch 5! (or GI, II, or BI, according to the magnitude of the loads), and reading the strains directly in micro-inches per inch (multiplied by a factor of 2, 5, or 10* if switches BI, II, or 8I are used).

The above discussion has been in connection with a specific form of the invention. It is obvious, however, that many changes may be made in this form without departing from the spirit of the invention. It is to'be understood, therefore, that the invention is not to be limited except as defined in the appended claims.

I claim:

1. Apparatus for measuring strains by means of strain gage resistance elements, comprising a plurality of sets of terminals adapted for connection with a like plurality of such strain gage elements, each set of terminals being connected with a resistance network which when connected with a strain gage element constitutes a Wheat stone bridge, each network having bridge input and output connections, and each network having an independent manually adjustable resistance for balancing'thebridge, a source of electrical energy and a circuit therefor including manually operable means for adjusting the curwhereby with the bridge input current adjusted by said current adjusting means into accordance with the galvanometer scale the bridges may be individually balanced by their respective adjustable resistances and thereafter quantitative measurements made of strains applied to the connected strain gage elements, and means, including a switch, connected between the source circuit and the galvanometer circuit, for unbalancing by a predetermined amount when said switch is closed any bridge that is selected by said manually operable switch means.

2. Apparatus for measuring strains by means I of strain gage resistance elements and compensating resistance elements, comprising a plurality of sets of terminals, each set being adapted for connection with one such strain gage element and one such compensating element,

each set of terminals being connected with a bridge resistance network which when connected with a, strain gage element and a compensating element constitutes a Wheatstone bridge, and each bridge network having an independent manually adjustable resistance for balancing the bridge, a'source of electrical energy and a circuit therefor including means for adjusting the current in the circuit, a galvanometer for indicating bridge output current value and a circuit therefor, the galvanometer circuit including a resistance network with switch means for varying the galvanometer range while maintaining the resistance of the galvanometer circuit approximately constant, and manually operable switch means for selectively connecting said source circuit with the input connections of any bridge network and for simultaneously connecting the galvanometer circuit with the output connections of the selected bridge network, whereby with the bridge input current adjusted by said current adjusting means into accordance with the galvanometer scale the bridges may be individually balanced by their respective adjustable resistance and thereafter direct quantitative measurements made of strains applied to the connected strain gage elements, and means, including a switch, connected between the source circuit and the galvanometer circuit, for unbalancing by a predetermined amount when said switch is closed any bridge that is selected by said manually operable switch means.

3. Apparatus for measuring strains by means of strain gage elements of substantially equal resistivity, comprising a plurality of sets of terminals adapted for connection with a like plurality of such strain gage elements, each set of terminals being connected with a resistance network which when connected with the strain gage element constitutes a Wheatstone bridge, all of said i0 networks having substantially equivalent resistance values, and each network having an independent manually adjustable resistance for balancing the bridge and input and output connections, a source of energy circuit including manually adjustable means for varying the current, a galvanometer for indicating bridge output current value, manually operable switch means for selectively connecting said source circuit with the input connections of'any network and for simultaneously connecting the galvanometer with the output connections of the selected network, and means, including a switch, connected between the source circuit and the galvanometer circuit, for unbalancing by a predetermined amount when said switch is closed any bridge that is selected by said manually operable switch means.

4. Apparatus for measuring strains by means of strain gage elements of substantially equal resistivity and compensating elements of substantially equal resistivity, comprising a plurality of sets of terminals each adapted for connection with one such strain gage element and one such compensating element, each set of terminals being connected with a resistance network which when said elements are connected therewith constitute a Wheatstone bridge, all of said networks having substantially equivalent resistance values, and each network having an independent manually adjustable resistance for balancing the bridge and input and output connections, a source of energy circuit including manually adjustable means for varying the current, a galvanometer for indicating bridge output current value, manually operable switch means for selectively connecting said source circuit with the input connections of any network and for simultaneously connecting the galvanometer with the output connections of the selected network, and means, including a switch, connected between the source circuit and the galvanometer circuit, for unbalancing by a predetermined amount when said switch is'closed any bridge that is selected by said manually operable switch means.

5. Apparatus for measuring strains by means of strain gage resistance elements, comprising a plurality of sets of terminals adapted for connection with a like plurality of such strain gage elements, each set of terminals being connected with a resistance network which when connected with a strain gage element constitutes a Wheatstone bridge, each network having bridge input and output connections, all of said networks having substantially the same resistance values and each network having an independent manually adjustable resistance for balancing the bridge, a source of electrical energy and a circuit therefor including manually operable means :for adjusting th current in the circuit, a galvanometer for indicating bridge output current value and a circuit therefor, manually operable switch means for selectively connecting said source circuit with the input connection of any network and for simultaneously connecting the galvanometer circuit with the output connections of the selected network, and a shunt circuit, including a resistance and a switch, across the source circuit and the galvanometer circuit, for unbalancing any selected bridge by a predetermined quantity whereby the source circuit current may be adjusted into accordance with the galvanometer scale.

6. Apparatus for measuring strains by means of strain gage resistance elements and compensating resistance elements, comprising a plurality of sets of terminals, each set being adapted for connection with one such strain gage element and one such compensating element, each set of terminals being connected with a bridge resistance network which when connected with a strain gage element and compensating element constitutes a Wheatstone bridge, each network having bridge input and output connections, all of said networks having substantially the same resistance values and each network having an independent manually adjustable resistance for balancing the bridge, a source of electrical energy and a circuit therefor including manually operable means for adjusting the current in the circuit, a galvanometer for indicating bridge output current value and a circuit therefor, manually operable switch means for selectively connecting said source circuit with the input connection of any network and for simultaneously connecting the galvanometer circuit with the output connections of the selected network, and a shunt circuit, including a resistance and a switch, across the source circuit and the galvanometer circuit, for unbalancing any selected bridge by a predetermined quantity whereby the source circuit current may be adjusted into accordance with the galvamometer scale.

7. Apparatus for measuring strains by means of strain gage elements of substantially equal resistivity, comprising a plurality of Wheatstone bridges of substantially equal resistance value with each bridge having one such gage element in'one arm thereof, each bridge having an independent manually adjustable resistance for bal- U ancing the bridge, means for providing a source of electrical potential of predetermined constant valueacross opposed bridge arm junctions, a galvanometer for indicating-bridge output current value and a circuit therefor, manually operable switch means for selectively connecting said galvanometer circuit to any selected one of said bridges across the remaining arm junctions thereof, said galvanometer circuit having a shunt thereacross of variable resistance value and having on each side of said shuntresistances of variable value, manually operable means for increasing the resistance of said shunt by predetermined increments and for simulaneously decreasing the resistance of each of said resistances of variable value by predetermined increments, whereby the range of the galvanometer may be varied by predetermined increments while maiintaining the resistance value of the galvanometer circuit approximately constant, and means, including a switch, connected between the source circuit and the galvanometer circuit, for unbalancing by a, predetermined amount when said switch is closed any bridge that is selected by said manually operable switch means.

8. In an instrument for measuring strains by means of strain gage resistance elements, a casing and means supported thereby comprising a galvanometer and circuit therefor, a manually operable selector switch, a battery and circuit therefor including adjustable means for regulating the current in the circuit, a plurality of sets of terminals adapted for connection with a like plurality of external strain gage resistance elements which may be attached to a structure to be tested, a resistance network connected with each set of terminals and which when such strain gage element is connected constitutes a Wheatstone bridge, all of said networks having substantially the same resistance values and each network having an independent manually adjustable resistance for balancing the bridge, said selector switch being arranged to connect the battery circuit across th current input bridge junctions of any selected one of said networks and to simultaneously connect the galvanometer circuit across the current output bridge junctions of the selected network whereby th galvanometer may be employed to indicate balance of any bridge and change of resistance of any of said strain gage resistance elements, and a resistance and switch means for connecting the same across the battery circuit and the galvanometer circuit whereby the means for adjusting the battery circuit current may be operated to calibrate the galvanometer in any selected position of said selector switch.

LOUIS D. S-TATHAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,460,530 Brown et al July 3, 1923 1,493,586 Wood May 13, 1924 1,877,739 Schneider Sept. 13, 1932; 2,047,343 Ward July 14, 1936 2,135,587 Lovelady Nov. 8, 1938 2,218,629 Swart Oct. 22, 1940 2,246,575 Coleman June 24, 1941 2,252,464 Kearns, Jr. et al. Aug. 12, 1941 2,360,886 Osterberg Oct. 24, 1944 2,389,615 Eder -1 Nov. 27, 1945 FOREIGN PATENTS Number Country Date 406,360 Great Britain Feb. 26, 1934 OTHER REFERENCES Publication entitled Strain Gages in Electropics, Dec. 1943, pages 106-11; 192-195. 

